DYNAMICS OF CYCLONES AND ANTICYCLONES. 151 



frequent renewal, and the rising fumes are patchy, thus allowing the core of the 

 cyclone to be often invisible, and they do not often show a well-defined core of 

 fumes. 



These experiments are sometimes difficult to repeat with any degree of certainty, 

 so much depends on the cross currents that happen to be in the room, which vary with 

 changes in the outside temperature, winds, etc., and inside heating. For this reason it 

 has been found advantageous to use some method of making a cross current that would 

 be completely under control. An ordinary screw-shaped fan of 75 cm. diameter does 

 very well for the purpose. Owing to a fan of this construction not delivering its air in 

 a uniform horizontal flow, but with an irregular somewhat spiral motion, it was placed 

 at a distance of about 1^ metres from the hot area, and into the space between the 

 fan and the hot area was fitted a horizontal surface, so as to cause the air to acquire a 

 horizontal flow before arriving at the hot area. With this apparatus no difficulty has 

 been experienced in repeating the experiments. 



These miniature cyclones illustrate many of the points referred to in the first 

 part of this paper. The slower rise of the air over the hot area when the circulation 

 is uniform across it, that is, so to speak, anticyclonic, and the collecting together of 

 the lower hot air, and the drawing of it up by the cyclone, and the power of the 

 whirling column to penetrate the higher regions, is well shown. These small cyclones 

 also illustrate the dependence of the spiral movements on the tangential current both 

 for their velocity as well as direction of rotation. They also illustrate that the direction 

 and the rate at which the cyclone as a whole travels is dependent on the direction and 

 velocity of this tangential current. 



If the cyclones be watched for a time whilst the draught in the room is not 

 constant in direction or force, it will be seen that the stronger the tangential current 

 the quicker the centre of depression moves along with it, and the more rapid is the 

 rate of rotation, and if the draught slows down and reverses, then the direction of 

 the movement of the centre of the cyclone also reverses, and it goes back on its track. 

 Further, if there is no tangential force, as when there are no draughts across the area, 

 then no cyclone is formed, and if formed during a temporary cross current it will die 

 away and cease to rotate when the cross current ceases. Again, cross currents of equal 

 force on opposite sides of hot areas keep the cyclone in one place and rapidly rotating. 



It is not necessary to make special apparatus for making these experimental 

 cyclones, as it will be found that a simple arrangement first tried is sufficient for the 

 purpose. It consisted of a large sheet of tin plate ; a thick sheet of iron would have 

 been better. This w r as covered with two or three sheets of thick paper well soaked in 

 water. This is placed over a stove or other means of heating it sufficiently to cause 

 steam to rise freely from its surface. When steaming freely the wet surface is removed 

 to a table, to be away from the draughts rising from the stove, and one side of the 

 hot surface is protected from cross draughts by means of a screen. When this is 

 done the cyclones will be seen travelling over the steaming surface. 



